DiskSpaceMonitorDescriptor currently contains the check

        public DiskSpace invoke(File f, VirtualChannel channel) throws IOException {
                long s = f.getUsableSpace();
                if(s<=0)    return null;
                return new DiskSpace(f.getCanonicalPath(), s);
        }

However, a return of 0 from File.getUsableSpace space is ambiguous. From the javadoc: "The number of available bytes on the partition or 0L if the abstract pathname does not name a partition. "

So a zero could mean "there is no partition" or that there are zero bytes left. In the /computer UI this shows up as "N/A" Free Disk Space.

Sample test program:

import java.io.File;

public class Hello {
    public static void main(String[] args) {
        File f = new File("/root/data/jenkins");
        System.out.println(f.getUsableSpace());
        System.out.println(f.getTotalSpace());
    }
}

On a totally full partition

# df  
Filesystem                                 1K-blocks      Used Available Use% Mounted on
zones/2222ef86-bdff-495f-a1f1-d47658d93769 107937943 107937943         0 100% /

The test program will output:

0
110528453632

I think this points to a solution: DiskSpaceMonitorDescriptor can also call getTotalSpace to determine what a zero from getUsableSpace really means.